Telecommunication systems

ABSTRACT

A telecommunications system having a format converter provides bi-directional communications between TDM signals on one side and ATM signals on the other. An ATM aggregate receives and transmits ATM signals, and a TDM interface receives and transmits TDM signals. The format converter has an ATM bus and a TDM bus connected to their respective interfaces, and service-specific adapters are connected between the ATM bus and the TDM bus.

This is a continuation application of patent application Ser. No.10/983,893 filed Nov. 9, 2004, now U.S. Pat. No. 7,974,274 which is acontinuation of patent application Ser. No. 09/581,476 filed Jun. 14,2000, now abandoned which is a nationalization of internationalapplication No. PCT/IE98/00103 filed Dec. 15, 1998, claiming benefit ofIrish patent application No. 970885 filed Dec. 15, 1997 and Irish patentapplication No. 5980709 filed Aug. 31, 1998, and hereby claims theforegoing priority to which it is entitled.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to telecommunication systems and moreparticularly to development of such systems to allow use of differentcommunication formats.

2. Description of the Related Art

The most common communication format for telecommunication systems istime division multiplexing (TDM) which is a continuous bit rate serviceconcatenated as multiples of 64 kb/s channels. The TDM technique isparticularly effective for voice communication because it isconnection-oriented, i.e. a connection link is established initially andis maintained throughout a communication session. Also, the speed andreliability are sufficient for voice.

However, TDM is not particularly effective for data traffic because itis inflexible and does not accommodate the “bursty” nature of data verywell. It was for this reason that packet-based techniques have beendeveloped for data communication. For example, the Internet Protocol(IP) technique is used for much Internet service provider (ISP) andlocal area network (LAN) communication. This technique usesvariable-length packets, each of which is independently routed. Thismeans that the technique is not connection-oriented and the bandwidthfor a call is not guaranteed for the duration of the call. The generalview is that voice subscribers would not tolerate the ensuingunpredictability of call quality. Also, the equipment required forprocessing to achieve the required speed for voice is quite complex.Therefore, while the IP technique is an approach which may be used tosome extent in the future for both voice and data telecommunicationnetworks, problems need to be overcome.

Another packet-based technique is the asynchronous transfer mode (ATM)technique. This has the advantages that it is packet-based and so canhandle bursty data traffic well, and is also connection-oriented. Inthis technique, data is transmitted in fixed-length packets which aretypically 53 Bytes in length, having a 48 Byte payload and a 5 Byteheader. The header comprises a virtual path identifier (VPI) and avirtual channel identifier (VCI). According to the ATM standard, thecell transfer technique is common to all services and therefore a singleATM stream of cells can include a wide range of different services.Because the cells have a fixed length, timing and control is morepredictable. On the other hand, packets in the IP technique are oftenmuch longer than ATM cells.

Packet-based techniques are radically different from the TDM techniqueand therefore it is not practical for a telecommunication operator tochange over in a short period of time. In order to introducepacket-based techniques such as ATM, adaptation processes have beendeveloped which are service-specific as different reconstructiontechniques are involved. This has led to a large degree of complexityand expense.

It is also known to provide a hybrid system in which TDM and ATM signalsare combined in a frame for communication over a link. Such anarrangement is described in PCT Patent Specification No. WO 97/18649(DSC Communications Corp.). While this arrangement is suitable forspecific application areas such as provision of services to a home, itdoes not appear to be efficient enough for telecommunication networks.This is particularly the case if a number of different services areinvolved.

European Patent Specification No. EP614324 (Nippon) describes a systemfor separate TDM/ATM and ATM/TDM conversion. However, this systemappears to be quite complex and to be difficult to scale up forincreased transaction volume capacity.

OBJECTS OF THE INVENTION

The present invention is therefore directed towards providing atelecommunication system to allow simpler growth in packet-basedtechnologies in a telecommunication network.

In this specification, the term “packet” is intended to mean bothvariable-length packets and fixed-length packets (cells). Also, in theremainder of the specification, the term “data” is intended to cover allservices, including voice.

SUMMARY OF THE INVENTION

According to the invention, there is provided a telecommunication systemcomprising:

-   -   a system TDM interface comprising means for receiving and        transmitting TDM data streams;    -   a system packet communication interface comprising means for        receiving and transmitting data packet streams; and    -   a format converter comprising means for performing        bi-directional conversion between the interfaces.

By providing bi-directional conversion, the invention allows conversionof telecommunications systems to technologies such as ATM in a gradualmanner. Of course, it also allows interfacing of packet-based systemswith TDM systems.

In one embodiment, the format converter comprises a TDM bus connected tothe TDM interface, a packet bus connected to the packet communicationinterface, and a conversion means connected between the two buses. Useof buses in this manner allows a great deal of flexibility inconfiguration of the conversion means, both in relation to services andhardware and software functionality.

Preferably, the conversion means comprises at least one service-specificadaptation module. Such modules operate efficiently for their particularservice, and this arrangement also allows modularity on a service basis.

In one embodiment, the system further comprises a system controllercomprising means for controlling operation of circuits in the system.This allows centralized general control. Preferably, the systemcontroller is connected to the packet bus. This allows utilization ofthe packet bus for control signals, thus allowing very effectivedistribution of control signals in a simple manner.

In one embodiment, the system controller comprises means fortransmitting and receiving system control signals via the packet bus tothe packet communication interface and the format converter, and meansfor transmitting and receiving system control signals to the TDMinterface via a separate TDM control signal link. Thus,conventional-format control signals are used for the TDM circuits.

In one embodiment, the control signals comprise cells such as ATM cells.

Preferably, the system packet communication interface and the formatconverter comprise means for adding an additional header to each cell todirect routing of the cells within the system. This is a simple andeffective way of efficiently distributing cells within the system. Forexample, the additional header may correlate a virtual circuit with anadaptation module.

In one embodiment, each adaptation module comprises a cell processorconnected to an adaptation circuit. By using a cell processor, there isflexibility in the range of cell-processing operations which may beperformed.

In another embodiment, each adaptation module further comprises acontrol processor, and the cell processor comprises means for routingcontrol signal cells to the control processor. By separating control andcell processing, the configuration is simple and easily controlled.

The cell processor routing means may comprise a segmentation andreassembly interface connected to a separate segmentation and reassemblycircuit, which is in turn connected to the control processor.

Ideally, the cell processor comprises means for stripping additionalheaders from cells as they are routed to the segmentation and reassemblycircuit.

In one embodiment, the cell processor comprises means for maintaining aplurality of output queues for routing of cells to the TDM bus, thequeues being maintained on a priority scheme according to VPI/VCIheaders. The allows excellent flexibility in traffic management.

Preferably, each cell processor comprises a mapping function foraddition of the additional headers.

Ideally, the cell processor comprises a dedicated ASIC. This allows veryefficient cell processing, and also modular construction of the systemat a lower level than the system interface and adaptation functions. Forexample, the system packet communication interface may comprise a cellprocessor and a bus interface, and the cell processor of the interfacemay comprise a similar configuration to the cell processor of eachadaptation module.

In one embodiment, the system packet communication interface furthercomprises a control processor, and the cell processor comprises meansfor routing control signal cells to the control processor.

According to another aspect, the invention provides a telecommunicationsystem comprising:

-   -   a TDM interface comprising means for receiving and transmitting        TDM data streams;    -   an ATM interface comprising means for receiving and transmitting        ATM data streams;    -   A format converter comprising a TDM bus connected to the TDM        interface, an ATM bus connected to the ATM interface, and at        least one service-specific adaptation module connected between        the buses; and    -   a system controller.

In one embodiment, the system controller is connected to the ATM bus andcomprises means for communicating with the ATM interface and eachadaptation module using cells.

In one embodiment, each adaptation module and the ATM interface comprisemeans for routing control signal cells with additional headers forinternal routing.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects of the present invention will be more clearlyunderstood from the following description of some embodiments thereof,given by way of example only with reference to the accompanyingdrawings.

FIGS. 1( a) and 1(b) are overview schematic representations of atelecommunication system of the invention.

FIG. 2 is a diagram illustrating construction of an adaptation module ofthe system.

FIG. 3 is a diagram illustrating a system ATM interface of the system.

FIG. 4 is a diagram illustrating an ASIC of the system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, there is illustrated a telecommunicationsystem 1. The purpose of the system 1 is to provide bi-directionalconversion between TDM signals on one side and packet-based signals onthe other. In this embodiment, the packet-based signals are ATM signals.Conversion is provided in a comprehensive manner whereby a wide varietyof different services may be incorporated in the incoming ATM or TDMsignals.

FIGS. 1( a) and 1(b) show overviews of the system 1. Referring initiallyto FIG. 1( a), the system 1 is shown at a high level. A bi-directionalTDM interface interfaces with external systems in the TDM network and abi-directional ATM interface interfaces with external systems in the ATMnetwork. Within the system 1, TDM signals are delivered into, andretrieved from, a TDM bus 4 and ATM signals are delivered onto andretrieved from an ATM bus 9. The buses are part of a format converterwhich also comprises multiple format conversion units connected betweenthem.

FIG. 1( a) illustrates an important aspect of the invention which is thefact that the TDM and ATM interfaces each communicate their relevantsignals within the system via a bus. This allows flexibility inconfiguration of the format converter as any desired number and type offormat conversion units may be connected between the TDM and ATM buses.These units may be service-specific. Therefore an important aspect ofthe invention is that the format conversion flexibility applies toallocation of services and also to allocation of circuits within thesystem for service and hardware utilization flexibility.

In more detail, and referring to FIG. 1( b), the system 1 comprises inthis embodiment three TDM interface units 2 operating at 45 Mb/s. Onlytwo are illustrated, for clarity. The interface units 2 are programmedto demultiplex received TDM signals and to multiplex outgoing TDMsignals. Differentiation of services in the signals is achieved byrecognition of framing patterns.

The format converter is indicated by the numeral 3 and comprises a TDMbus 4, adaptation modules 5 to 8, and an ATM bus 9. Thus the formatconversion units are service-specific adaptation units or modules,namely voice 5, frame relay 6, circuit emulation 7, and native ATM 8adaptation modules. Each adaptation module is programmed to extract therelevant incoming TDM or ATM signal for its service, to convert thesignal, and to route the converted signal onto the opposite bus foroutput at the relevant interface. In this embodiment, one module perservice is used, however, the flexibility allowed by the architectureallows multiple modules per service.

The ATM interface comprises an ATM aggregate 10 operating at 155 Mb/s.

The manner in which the adaptation modules extract the relevant signalsis according to control signals from a system controller 11. Thecontroller 11 directs control signals via the ATM bus 9 for the ATMaggregate 10 and for the adaptation modules. The communications protocolis ATM, with specific internal headers. This has been found to be aparticularly convenient form of communication. The controller 11communicates with the TDM interfaces via a separate dedicated link 12because the latter does not have access to ATM signals.

Referring now to FIG. 2, construction of an individual adaptation moduleis illustrated, in this case the voice adaptation module 5. The module 5comprises an ASIC cell processor 20 having inter alia a routing function21 and a mapping function 22. There is an ATM bus interface 25connecting it to the ATM bus 9. On the other side, the module 5 isconnected via a TDM/ATM adaptation device 26 and a TDM bus interface 27to the TDM bus 4. For internal control, the module 5 comprises aSegmentation and Reassembly device (SAR) 28 connected to the cellprocessor 20 and on the other side to a control processor 29. Thecontrol processor 29 is connected by a control bus 30 to the SAR 28, thecell processor 20, the adaptation circuit 26, and to the TDM businterface 27.

As illustrated in FIG. 3 the ATM aggregate 10 has the configuration ofan ATM bus interface 40 (connected to the ATM bus 9), a cell processor41, a SAR 42, and a control processor 43. This configuration may beregarded as a general modular circuit for the system as it forms part ofeach of the modules 5 to 8. Within the cell processor 41, there is amapping function 44 and a routing function 45. The latter maintainscontrol signal queues (CSQ) and traffic queues (TQ).

In operation, when a cell is received at the ATM aggregate 10, theVPI/VCI is used by the mapping function 44 in the cell processor 41 todetermine the relevant circuit. The mapping function 44 adds anadditional 4 Byte header accordingly. This header allows the relevantadaptation module ATM bus interface 25 to extract the cell and pass itto the routing function 21 of the associated cell processor 20. Wherethere are a number of adaptation modules for each service the additionalheader specifies the particular ATM bus interface 25 by correlation ofthe virtual circuit with a physical circuit. This is important forinternal traffic management.

The system controller 11 also transmits cells, namely control signalcells to the ATM bus 9. Again, an additional header is used for routing.The routing function reads the additional header 21 to determine whethereach cell is part of a control signal or is a traffic cell. If part of acontrol signal, it strips off the additional header and writes theremainder (in conventional 53 Byte format) to an output control signalqueue (CSQ). Thus, the SAR 28 only handles conventional-format cells,which it converts to messages recognized by the control processor 29.The control processor 29 acts upon these signals to transmit bus controlsignals to the various parts of the adaptation module. For example, itmay set the manner in which the TDM bus interface 27 extracts TDMsignals from the TDM bus 4. The control processor 29 transmits responsessuch as set-up verification signals, failure condition information etc.,to the system controller 11 via the SAR 28 and the mapping function 22.This allows the system controller to monitor status of the system 1.

If a received cell is part of the system traffic, the additional 4-Byteheader is again stripped off, but the remaining cell is placed in arelevant traffic queue (TQ). The particular queue is chosen according tothe VPI/VCI and the additional header to take account of criteria suchas service priority. The cells are converted by the adaptation circuit26, which is pre-configured for and specific to the particular service.The TDM bus interface 27 in turn places the TDM signals on the TDM bus4.

In the other direction, the manner in which TDM signals are extracted iscontrolled by the control processor 29 according to control signals fromthe system controller 11. Examples of extraction conditions aretimeslots to be extracted or whether echo cancellation is required.

These signals are converted by the adaptation circuit 26, which outputsATM cells to the mapping function 22. This function has a look-up tablewhich inserts the additional header for system use. They may specify,for example, a particular ATM aggregate 10 if there are several of them.

The output cell is then directed to the ATM bus interface 25 and thenonto the ATM bus 9. It will be noted that the incoming cells from theTDM side do not include control signal cells—all being traffic cells.The cells are retrieved by the ATM bus interface 40 of the ATM aggregate10, which in turn directs them to the routing function 45. As controlsignal cells are also received in this manner, the routing function 45maintains both traffic queues TQ and control signal queues CSQ.

The cell processors 20 and 41 each comprise an ASIC 50, illustrated inFIG. 4. Cell rates of up to 373 K cells per second are handled. Cellsfrom the line (controlled rate flow) are received by a UTOPIA lineinterface 51, which passes them to a mapping circuit 52 which performsthe VPI/VCI mapping. A policing circuit 53 performs usage parametercontrol (UPC) by which cells are admitted based on programmed ratecontrol limits. Statistics are kept on the number of cells admitted,number of bad cells, number of policed cells, and the number of disabledcells. The cells are then passed to a backplane interface 54 fortransfer, for example, onto the ATM bus interface 25.

In the other direction, cells are received by the backplane interface 54and passed for queue control to a queuing circuit 55. This uses a cellRAM controller 56 to store queued cells externally, and an SRAMcontroller 57 to store queue configuration data. The queues aremaintained for the line interface 51, and for a SAR interface 58. Thus,the queuing circuit 55 together with the cell RAM controller 56 and theSRAM controller 57 implement the routing function. As will beappreciated from FIGS. 2 to 4, the queuing is primarily uni-directional.In the case of the adaptation modules, the cells from the TDM/ATMadaptation circuit 26 arrive at a controlled rate and queuing is notnecessary. However, in the opposite direction, there is less controlover the rate of the incoming traffic cells, and control signal cellsare also included. In the case of the ATM aggregate 10, there issynchronization with the ATM line interface 46 and queuing is notnecessary. However, in the opposite direction there is less control asvarying numbers of adaptation modules operate at full capacity, and ofcourse control signal cells are also included.

A processor interface 59 with a configuration and status circuit 60allow processor access to, for example, configure the SRAM initially forsuch things as queue control. However, most on-going control isperformed using ATM cells via the SAR interface 58.

The processor interface 59 may also receive and transmit cells to/fromany of the local or remote cell interfaces in the system, and has theadditional ability to insert a CRC-10 check sum on cells it transmitsand to verify a CRC-10 check sum on cells it receives. This is usefulfor generating OAM cells and for verifying the validity of OAM cells.The processor interface 59 is also used for gathering statisticalinformation on the number of ATM cells in the system.

It will be appreciated that the invention provides for relatively simplemigration of telecommunication networks to ATM transmission. The systemhas a large degree of flexibility provided by the array of differentadaptation modules. This allows multiple services to interwork over highspeed transmission paths between two network types by converting eachservice in an appropriate adaptation function.

The invention is not limited to the embodiments hereinbefore described,but may be varied in construction and detail within the scope of theclaims. For example, while the embodiment illustrated has an ATMinterface, this may instead be for any packet-based communicationtechnique such as IP. In this case the bus connected to this interfacehandles packets. However, it is still preferred that fixed length packet(cells) are used for internal control and data rather thanvariable-length packets. This is because such packets generally allowmore predictable timing of internal signal routing. The packetcommunication interface would perform conversion between the externalvariable-length and the internal fixed-length packet environments. Forexample, a segmentation and reassembly device may be used.

What is claimed is:
 1. A format converting device for performingbi-directional conversion within a telecommunication system comprising:a time division multiplexing (TDM) bus connected to a TDM interface andconfigured to receive and transmit data streams; a packet bus connectedto a packet communications interface and configured to receive andtransmit data packet streams; a separate segmentation and reassemblycircuit; and a service-specific adaptation unit connected between theTDM bus and the packet bus, the service-specific adaptation unitincluding a cell processor connected to an adaptation circuit and acontrol processor, the cell processor having a routing function thatroutes system control signal cells to the control processor through asegmentation and reassembly interface connected to the separatesegmentation and reassembly circuit.
 2. The format converting device asclaimed in claim 1, wherein the packet bus is connected to a systemcontroller within the telecommunication system.
 3. The format convertingdevice as claimed in claim 2, wherein the system controller isconfigured to transmit and receive system control signals via the packetbus to the format converting device.
 4. The format converting device asclaimed in claim 1, wherein the cell processor strips additional headersfrom control signals as the control signals are routed to thesegmentation and reassembly circuit.
 5. The format converting device asclaimed in claim 4, wherein the control signals are cells, said devicebeing configured to add an additional header to each control signal cellto direct routing of the control signal cells within thetelecommunication system.
 6. The format converting device as claimed inclaim 5, wherein the bi-directional conversion performed by the formatconverting device is specific to voice.
 7. The format converting deviceas claimed in claim 5, wherein the bi-directional conversion performedby the format converting device is specific to frame relay.
 8. Theformat converting device as claimed in claim 5, wherein thebi-directional conversion performed by the format converting device isspecific to circuit emulation.
 9. The format converting device asclaimed in claim 5, wherein the bi-directional conversion performed bythe format converting device is specific to native asynchronous transfermode (ATM).
 10. The format converting device as claimed in claim 5,wherein the cell processor is configured to maintain a plurality ofoutput queues for routing of the control signal cells to the TDM bus,the queues being maintained on a priority scheme according to headersthat include a virtual path identifier (VPI) and a virtual channelidentifier (VCI).
 11. The format converting device as claimed in claim5, wherein the cell processor includes a mapping function for adding theadditional headers to the control signal cells, respectively.
 12. Theformat converting device as claimed in claim 1, wherein the cellprocessor includes a dedicated application-specific integrated circuit(ASIC).
 13. A format converting device for performing bi-directionalconversion between a time division multiplexing (TDM) interface and apacket communication interface within a telecommunication systemcomprising: a TDM bus connected to the TDM interface; a packet busconnected to the packet communication interface; a service-specificadaptation unit connected to and between each of the TDM bus and thepacket bus, the service-specific adaptation unit configured to extractat least one TDM signal received from the TDM interface via the TDM busand to convert the at least one TDM signal to a data packet signal andoutput the converted data packet signal to the packet bus for routing tothe packet communication interface; the service-specific adaptation unitfurther configured to extract at least one data packet signal receivedfrom the packet communication interface via the packet bus, convert theat least one data packet signal to a TDM signal and output the convertedTDM signal to the TDM bus for routing to the TDM interface; theservice-specific adaptation unit including a control processor and acell processor; the cell processor being coupled to the packet busthrough a packet bus interface and being coupled to the TDM bus throughan adaptation circuit and TDM bus interface; and the cell processorincluding a segmentation and reassembly interface connected to aseparate segmentation and reassembly circuit, the cell processor beingconfigured to route control signal cells to the control processorthrough the segmentation and reassembly interface.
 14. The formatconverting device as claimed in claim 13, wherein the cell processor isconfigured to maintain a plurality of output queues for routing of thecontrol signal cells to the TDM bus, the queues being maintained on apriority scheme according to headers that include a virtual pathidentifier (VPI) and a virtual channel identifier (VCI).
 15. The formatconverting device as claimed in claim 13, wherein the service-specificadaptation unit includes a system control processor for transmitting andreceiving control signal cells.
 16. The format converting device asclaimed in claim 13, wherein the cell processor includes a mappingfunction for adding additional headers to the control signal cells. 17.A format converting method comprising: receiving and transmitting timedivision multiplexing (TDM) data streams over a TDM bus of a formatconverter, the TOM bus being connected to system TDM interface;receiving and transmitting data packet streams that include IP packetsover a packet bus of the format converter, the packet bus beingconnected to a system packet communication interface; performingbi-directional conversion using the format converter, the formatconverter including a service-specific adaptation module connectedbetween the TDM bus and the packet bus; extracting, by theservice-specific adaptation module, at least one TDM signal receivedfrom the TDM interface via the TOM bus, converting the at least one TDMsignal to a data packet signal, and outputting the converted data packetsignal to the packet bus for routing to the packet communicationinterface; further extracting, by the service-specific adaptationmodule, at least one data packet signal received from the packetcommunication interface via the packet bus, converting the at least onedata packet signal to a TOM signal, and outputting the converted TDMsignal to the TDM bus for routing to the TDM interface; and transmittingand receiving control signals via the packet bus to the formatconverter, and transmitting and receiving control signals over the TDMbus via a separate dedicated TDM control signal link.
 18. The method asclaimed in claim 17, further comprising the format converter maintaininga plurality of output queues for routing of cells to the TDM bus, thequeues being maintained on a priority scheme according to headers thatinclude a virtual path identifier (VPI) and a virtual channel identifier(VCI).
 19. The method as claimed in claim 17, wherein the formatconverter includes a control processor and a cell processor including asegmentation and reassembly interface connected to a separatesegmentation and reassembly circuit, the method further including thestep of the cell processor routing control signal cells to the controlprocessor through the segmentation and reassembly interface.